The methods for treating soils contaminated with heavy metals known to the skilled-in-the-art are based on either washing the soil with solutions which change metal valency and/or making metals insoluble or detoxifying and immobilizing the metals within the soil matrix.
The known soil washing methods suffer many drawbacks because a substantial portion of the heavy metals in the soil, particularly chromium, are bound with organic soil components and are difficult to release. Thus, heavy metals in the soil are typically bound with organic matter (e.g. humic acids) and sometimes are poorly extracted unless the organic compounds are destroyed. These complexes are unstable under acidic or alkaline conditions when hydrolysis of organic substances occurs. To fully extract heavy metals from the soil, it is usually necessary to add chemical reagents to lower pH to 2-3, wash the soil by 2-3 volumes of elutriate to ensure full metal recovery and then adjust pH to 7.0-7.5. This is a costly procedure for industrial use. Also, these methods usually require extensive use of expensive reagents, e.g. acids or alkali, etc.
There is a method for heavy metal contaminated soil treatment (the technology is filed in the United States Environmental Protection Agency database VISITT under vendor name IT Corp. Mar. 10, 1993). Heavy metal are extracted from the soil slurry by contact with hydrochloric acid which then is drawn-off, two-stage the water washing and final caustic wash. An important drawback of the method is foaming of the slurried soil.
Scientific Ecology Group, Inc. has developed a process for removing heavy metal contaminants from soil by using an extraction solution containing sufficient cations to displace the contaminant from soil (the technology is filed in the EPA database VISITT Mar. 31, 1993). The main disadvantage of the method is that soil has low permeability and thus is not substantially dewatered.
There is known a resin-in-pulp technology to extract metal contaminants (the technology is filed in the EPA database VISITT under the trade name "In-Pulp Decontamination of Contaminated Soil" Feb. 12, 1993). This technology uses an adsorbent such as ion exchange resin or active carbon to extract the dissolved metals. The shortcoming of the technology is the necessity to separate soil from pulp particles which is a very costly process. Also, the synthetic resins used for the process are very expensive.
There exists a technology registered in the EPA database VISITT under the trade name "TerraMet Heavy Metal Removal Technology" Feb. 11, 1993 which represents an extractive process which leaches heavy metals from contaminated soil with a proprietary aqueous leaching solution and subsequent metal ion removal using liquid ion exchange, resin ion exchange or reduction. The main disadvantage of the method is the presence of high levels of surfactants. Additionally, this method suffers several of the same drawbacks as the above discussed methods.
Further, known soil immobilization and cement encapsulation methods generally result in substantial increase of the treated soil volume.
U.S. Pat. No. 5,304,710 describes a method for treating chromium ore waste contaminated soils by detoxification, fixation, immobilization and, stabilization. The drawbacks of the method include its use of expensive ferrous sulfate and a substantial increase in treated soil volume caused by forming crystalhydrates in course of the fixation process.
U.S. Pat. No. 4,629,509 describes a method for treating cadmium and lead contaminated solid residue with calcium sulfide to reduce leaching of cadmium and lead from said residue to environmentally acceptable levels. A particularly convenient method involves addition of dry lime and an aqueous solution of an inorganic salt supplying sulfide, especially an alkali metal sulfide. The main disadvantage of the method is that the amount of reagent additives strongly depends on heavy metal content and it is impractical for heavy contaminated soils, while also being too complicated for use with lower concentration of metals.
U.S. Pat. No. 4,744,829 describes a process for production of agglomerates which are resistant to leaching out, from finely divided waste materials by dissolving a hydraulic binding agent under a condition of forced mixing, combining the resulting aqueous colloidal solution with finely divided waste materials, agglomerating by mixing, and then hardening. The main drawback of the method is that the agglomerates hardening is very slow under its conditions of low temperatures thus requiring additional heating which adds to the cost of the process.
U.S. Pat. No. 4,005,009 describes a method of treating waste water containing surfactants and heavy metals such as Cd, Cr, Mn, Hg, etc. by the flocculation process with a cationic macromolecular reagent at a dosage ranging from 1 to 50 ppm. The cationic macromolecular reagent for use in the method includes polyacrylamine as well as denatured substances thereof, polyimine, etc. to facilitate adsorption of the aggregate substance, surfactants and heavy metals by foams. The process of binding the heavy metals by this patent is based on chemical adsorption and is insufficiently effective for higher concentration of heavy metals. In particular, this method is impractical for removal of heavy metals contained in the soils where they are tightly bound with organic material.
U.S. Pat. No. 4,943,377 discloses a method for removal of dissolved heavy metals from waste oils, industrial wastewater, or polar solvent by metallic polythiocarbonate formation. This method when used for soil treatment requires substantial dilution which results in need for large volumes of the reactors and the use of a very high dosage of reagents.
For solidification of sludge or semi-liquid wastes, some polymeric materials may be added as a coagulation/cross-linking aid.
U.S. Pat. No. 4,338,134 describes a process for binding waste liquor or sludge, in which a hydraulic mortar is added and the mixture is subjected to a milling operation and then allowed to set. Optionally, from 0.5 to 20% by weight of a polymerisable resin such as a urea-formaldehyde or a melamine-formaldehyde or an acrylic resin are added. The drawback of the method is the high cost of the polymerisable resin and bulking of the final product.
The present invention provides a combination of washing and encapsulation techniques using solutions of anionic and when appropriate cationic synthetic organic flocculants which avoid the main drawbacks mentioned above. This allows one to minimize the use of reagents compared with the washing techniques, and does not result in the increase of treated soil volume compared with the known encapsulation methods.